A system of controlling the flow rate of pressurized liquid to a control valve has been proposed in the art in order to change the force needed for steering in a power steering system. In addition, a liquid pressure reaction force chamber system is known in which, as disclosed by Japanese Patent Application Publication No. 43946/1980, the equivalent spring constant between input and output shafts in a power steering mechanism is controlled by the reaction force of the liquid pressure reaction force chamber. The relative displacement of the input and output shafts is adjusted to determine the degree of opening of the control valve in order to change the steering force.
In these conventional systems, the degree of opening of the orifice in a liquid pressure circuit is changed to control the flow rate of pressurized liquid and thus set the reaction force. The degree of opening of the orifice is controlled by an actuator such as a linear solenoid, stepping motor, or DC servo motor.
In general, during the driving of a vehicle, the force needed for steering is affected by a variety of external factors. Accordingly, in order to give a suitable steering feeling to the driver of a vehicle equipped with a power steering system, it is necessary to control the degree of opening of the orifice in the liquid pressure system in accordance with these external factors.
The control of the orifice must take into account the weight of the vehicle, the number of people in the vehicle, the amount of load including the weight of cargo in the vehicle, the steering hydraulic pressure system, the characteristics of the mechanical system, and the design.
Heretofore, circuit constants for determining the relationships between the vehicle speed, orifice control data, and the configuration of the orifice have had to be finely adjusted. With the systems of the prior art, a manufacturer supplying control devices to customers must design control devices separately for the above-described external factors. In addition, adjustment is required in the tuning of the steering system to the vehicle which increases the manufacturing cost and extends the period for development.